The present invention relates to a method and apparatus for applying a metallic coating to threaded end sections of plastic pipes that can be forcibly screwed to each other.
For several years in the production of petroleum and natural gas, it has been necessary, as the deposits of the easily exploitable fields have been increasingly exhausted, to drill to progressively greater depths. Also fields which were classified as difficult were put into production, in which connection the percentage of strongly corrosive elements in the fluid to be conveyed, such as CO.sub.2, H.sub.2 S and chlorides, alone and in combination, become increasingly greater. In addition to this, it has also become customary in petroleum production to force salt water, obtained upon the separation of the oil, partially or entirely back into the borehole in order to maintain a necessary reservoir pressure. Pipes made of ordinary carbon steel cannot withstand the resulting massive corrosive attack; there are a large number of proposals for the solution of this problem.
One of the proposals to solve the foregoing corrosion problem is to line the entire inside of a pipe with a corrosion-resistant material, for instance plastic, or to form a pipe from a corrosion resistant high-alloy steel having a high content of chromium, nickel or similar elements. The lining of carbon steel pipes with a so-called in liner of plastic, however, is very difficult. This is since, as a result of the very different coefficients of thermal expansion, it is difficult to clamp the two pipes together; and the ends, flanged in most cases, of the inward-drawn plastic pipes tend to crack upon changing temperature stresses The corrosion-resistant high-alloy steel pipes used as an alternative are very expensive since the pipes have a high alloy content of very expensive elements such as chromium and nickel.
Against this background, the use of pure plastic pipes, in particular fiber-reinforced plastic pipes, has become increasingly interesting since they are considerably cheaper than pipes of high alloy steel and have a high resistance to corrosion by the corrosive fluids that occur upon the production of oil. Since it is customary in the production of petroleum and natural gas to reuse the pipes insofar as possible, their ends are provided with a threaded section so that they can be screwed either directly or via a socket to each other. This manner of connection is used in the same way also in the case of plastic pipes. In this connection it is found that, particularly in the case of fiber-reinforced plastic pipes, screwing is in many cases not possible without damaging the thread. In some cases, even the pipe itself would break. The mechanism of the strong clamping of the two threaded surfaces that are pressed against each other has still not been entirely explained; however, there are many indications that, as a result of the high pressure per unit area in the threads, the bonding forces between the plastic surfaces that rest against each other increase greatly despite the lubricant used The torque required for unscrewing a pipe coupling increases to the same extent, so that, above a critical torque, damage to the threaded section is inevitable.
In order to overcome this problem, it has therefore been proposed (EP 0 292 998) to plate the threaded sections with a layer of metal In this case, in an initial step the threaded section is copper-plated or nickel-plated by a currentless wet process, whereupon a layer of copper or nickel is applied by electroplating. The first layer applied is in most cases very thin, about 0.3 .mu., since it merely serves to make the threaded section electrically conductive for the subsequent electroplating. The layer applied by electroplating is within the range of 2 to 5 .mu. and up to at most 50 .mu., since above that value the adherence of the applied layer decreases considerably.
The method described has the great disadvantage that the plating requires a large amount of time, i.e. on the order of 90 minutes including the necessary preliminary treatment, and is therefore very expensive. Furthermore, the method cannot be introduced directly into the production line due to the long processing time, but must be carried out in a bypass procedure. In view of the increasing importance of protecting the environment, it must furthermore be pointed out that the disposal of the corrosive fluids used for the chemical baths is difficult and also increasingly expensive. Furthermore, a lengthy approval procedure is required for installing such baths, particularly in critical water-catchment regions.